The compressor section of a conventional gas turbine engine usually includes a diffuser located downstream of the centrifugal compressor turbines and impeller, and upstream of the combustor. The function of a diffuser is to reduce the velocity of the compressed air and simultaneously increase the static pressure thereby preparing the air for entry into the combustor at a low velocity and high pressure. High-pressure low velocity air presented to the combustor section is essential for proper fuel mixing and efficient combustion.
Gas turbine engines that include a centrifugal impeller as the high-pressure stage of the compressor are suitable for application of the present invention. Centrifugal impellers are used generally in smaller gas turbine engines. A compressor section may include axial or mixed flow compressor stages with the centrifugal impeller as the high-pressure section or alternatively a low-pressure impeller and high-pressure impeller may be joined in series.
The centrifugal compressor impeller draws air axially from a low diameter. Rotation of the impeller increases the velocity of the air flow as the input air is directed over impeller vanes to flow in a radially outward direction under centrifugal force. In order to redirect the radial flow of air exiting the impeller to an annular axial flow for annular presentation to the combustor, a diffuser assembly is provided to redirect the air from radial to axial flow and to reduce the velocity and increase static pressure.
A conventional diffuser assembly generally comprises a machined ring which surrounds the periphery of the impeller for capturing the radial flow of air and redirecting it through generally tangential orifices into an array of separate diffuser tubes. The diffuser tubes are generally horn-shaped with an increasing internal cross-section and bend to direct air from the radial to axial direction. The diffuser tubes are formed of sheet metal with a longitudinal seam. The narrow end of the diffuser tubes are brazed or mechanically connected to the central ring and have an increasing cross-section rearwardly. As a result, the narrow stream of air at high pressure taken into the orifices in the ring are expanded in volume as the air travels axially through the diffuser tubes. The increase in air volume results in a reduced velocity and corresponding increase in static pressure, (i.e.: kinetic energy is converted to pressure energy, where total energy of a fluid flow remains constant being the sum of the pressure energy, potential energy and kinetic energy, Bernoulli theorem)
Fabrication of the conventional diffuser with individual tubes is extremely complex since the tubes have a flared internal pathway that curves from a generally radial tangential direction to an axial rearward direction. Each tube must be manufactured to close tolerances individually and then assembled to the machined central ring. Complex tooling and labour intensive manufacturing procedures result in a relatively high cost for preparation of the diffusers.
During engine operation, diffusers often cause problems resulting from the vibration of the individual diffuser tubes. Vibration can cause a reduction in service life due to metal fatigue, causes instability in the engine compressed air flow, and adds to the engine noise. To remedy vibration difficulties, the diffuser tubes may be joined together or may be balanced during routine maintenance. However, such operations are labour intensive, and involve costly downtime for the engine. From an aerodynamic standpoint, the joining of individual diffuser tubes to the machined central ring results in interior surface transitions that inevitably effect the efficiency of the engine detrimentally due to the high velocity of air flow. On the interior of the tube as it joins the orifice in the ring, there is a step or transition caused by manufacturing tolerances in the assembly and brazing procedures. Since the air in this section flows at supersonic velocity, even minute disturbances in air flow and increases in drag as the air flows over such transitions can result in very high losses in efficiency.
In general, the design of diffusers is not optimal since their complex structure requires a compromise between the desired aerodynamic properties and the practical limits of manufacturing procedures. For example, the orifices in the impeller surrounding ring are limited in shape to cylindrical bores or conical bores due to the limits of economical drilling procedures. To provide elliptical holes for example, would involve prohibitively high costs in preparation and quality control. The shape of the diffuser pipes themselves is also limited by the practical considerations of forming their complex geometry. In general, the diffuser tubes are made in a conical shape and bent to their helical final shape prior to brazing. Whether or not this conical configuration is optimal for aerodynamic efficiency becomes secondary to the practical considerations of economical manufacturing.
Diffuser designs incorporating multiple diffuser tubes have the advantage that oil lines can easily be passed between adjacent tubes through the diffuser. As a result, bearings can be located adjacent the combustor area to support the high pressure shaft where loading is most critical. The disadvantages inherent in a complex diffuser design are justified by the advantages inherent in centrifugal compressor efficiency and preferred bearing locations, particularly in small engines.
Due to the radial extent of the combined centrifugal compressor and diffuser, together with any external bypass ducts, the diameter of the diffuser assembly contributes significantly to the overall diameter of the entire engine. Reduction in the diameter of the diffuser assembly can result in reduction of engine diameter which significantly effects the drag and fuel efficiency of an aircraft.
It is an object of the invention to provide a diffuser assembly which significantly reduces the tooling and manufacturing costs associated with prior art diffuser assemblies, thereby reducing costs and manufacturing time.
It is an object of the invention to significantly reduce the losses in efficiency that result from locating transitions in areas of the diffuser passages carrying high velocity air flow.
It is an object of the invention to reduce the vibration difficulties and the number of parts resulting from use of multiple independent diffuser tubes.
It is an object of the invention to rationalize the various components of a conventional diffuser design and adjacent engine structures into a more compact structurally integral robust unit, preferably with reduced overall diameter.
Further objects of the invention will be apparent from review of the disclosure and description of the invention below.